The CHD5 chromatin remodeler plays a critical role in tumor suppression in humans. Loss of the CHD5 locus is associated with formation of a wide range of tumors, including colorectal cancer, leukemia, lung cancer, and neuroblastoma. Furthermore, loss of CHD5 expression is strongly correlated with poor prognosis in a variety of tumors whereas ectopic expression studies have shown that increased expression of CHD5 in tumor cells can suppress their growth. The ability of CHD5 to suppress the tumorigenicity of cancer cells suggests that medical strategies that augment CHD5 function will suppress the growth of tumors associated with loss or reduced expression of CHD5. Development of such medical strategies requires an in depth understanding of the mechanism(s) by which CHD5 acts. Our long-term goal is therefore to understand how CHD5 suppresses tumor formation and development and to use this understanding to develop novel anti-tumor strategies. Phylogenetic analysis reveals that CHD5 proteins are found only in vertebrates, which in turn suggests a conserved function for the CHD5 remodeler in this lineage. This proposal examines the role of the zebrafish CHD5 gene chd5 in tumor suppression and gene expression in zebrafish with the goal of establishing this powerful vertebrate system as a suitable context for determining the mechanism of tumor suppression by the CHD5 chromatin remodeler. Preliminary data show that chd5 is expressed in a similar fashion to mammalian CHD5 in adults and reveal a novel pattern of expression in early embryos as well as a role for chd5 in embryogenesis. The first aim of the proposal uses transgenic lines expressing mutated versions of chd5 to establish whether chd5 acts as a tumor suppressor in zebrafish as it does in mammals. The second aim is a biochemical screen for co-factors of Chd5 protein that will help reveal how Chd5 acts and that will also be candidates for factors that contribute to CHD5-dependent tumor suppression. The third aim employs an RNA-seq strategy to identify genes that exhibit chd5-dependent expression to identify target genes that mediate the ability of chd5 to suppress tumor formation. Completion of our experimental aims will establish zebrafish as a model for characterization of how CHD5 remodelers suppress tumors that will serve as a vital complement to the mouse model. In particular, our proposed studies will enable us undertake the type of comprehensive chemical genetic screens that are practical in zebrafish to identify compounds that suppress chd5-dependent defects and thereby identify lead compounds for development for pharmaceuticals to treat CHD5-dependent tumors. Similarly, completion of the proposed aims will enable us to use the formidable forward and reverse genetic tools available in zebrafish to identify additional epigenetic machinery that contributes to CHD5-mediated tumor suppression. Identification of such machinery will therefore reveal additional targets for anti-tumor therapies for CHD5-dependent tumors.